archetype flows on the board list the agents the router sends to, and the docs say a switch-off key outlives its flow

swarm-queue-client/src/agent_status.rs

@@ -20,3 +20,34 @@ pub async fn open_or_create(
     )
     .await
 }
+
+/// The agents of the swarm: the agent names in the bucket's keys
+/// (`<hive>.<agent>`), read from the stream's subject list. This is the
+/// roster the notification router sends to.
+///
+/// # Errors
+///
+/// The bucket's stream cannot be opened or its subjects listed.
+#[cfg(feature = "kv")]
+pub async fn roster(
+    client: &async_nats::Client,
+) -> Result<std::collections::BTreeSet<String>, Box<dyn std::error::Error + Send + Sync>> {
+    use futures_util::TryStreamExt as _;
+
+    let bucket = swarm_wire::agent_status::BUCKET;
+    let js = async_nats::jetstream::new(client.clone());
+    let stream = js.get_stream(format!("KV_{bucket}")).await?;
+    let subjects: Vec<(String, usize)> = stream
+        .info_with_subjects(format!("$KV.{bucket}.>"))
+        .await?
+        .try_collect()
+        .await?;
+    let prefix = format!("$KV.{bucket}.");
+    Ok(subjects
+        .into_iter()
+        .filter_map(|(subject, _)| {
+            let key = subject.strip_prefix(&prefix)?;
+            swarm_wire::agent_status::split_key(key).map(|(_, agent)| agent.to_owned())
+        })
+        .collect())
+}